Method for processing an olefin-containing product stream

ABSTRACT

A method is described for processing an olefin-containing product stream ( 1 ) that contains, besides ethylene and propylene, longer-chain olefins and compounds of hydrocarbons with oxygen (oxygenates). Such a product stream (I) can occur in particular in olefin synthesis from methanol. The product stream is dewatered in successive process steps, optionally compressed in several steps and dried. Then it is subjected to fractionation. For removal of the undesired oxygenates it is proposed that the oxygenates be washed out of the gaseous product stream after the compression steps and before the drying step in a column ( 12 ) with methanol.

This application claims benefit of the filing date of German PatentApplication No. 101 50 480.2, filed Oct. 16, 2001.

FIELD OF THE INVENTION

This invention concerns a method for processing an olefin-containingproduct stream that contains, besides ethylene and propylene, longerchain olefins and compounds of hydrocarbons with oxygen (oxygenates),where the product stream is dewatered in successive process steps,optionally compressed in several steps and dried, and then sent tofractionation.

BACKGROUND OF THE INVENTION

Producing olefins from methanol have been considered an interestingalternative to the traditional production of olefins from petroleum.Methanol is considered to be a readily stored and managed intermediateproduct for utilization of hitherto unused natural gas. Thus, theincreasing demands for olefins on the world market could also be servedby using very cheap methane. For this reason, processes are beingdeveloped that produce short-chain olefins from methanol. Such processesoperate, for example, catalytically according to the overall equation2CH₃OH→C₂H₄+2H₂O. Besides the desired olefins, ethylene and propylene,longer-chain olefins and, especially, undesired compounds ofhydrocarbons with oxygen (oxygenates) such as alcohols, ketones andorganic acids also form. For this reason a costly secondary purificationof the reaction product is necessary. One oxygenate that is particularlyto be taken into account is dimethyl ether (DME), since it is one of thelightest oxygenates and behaves similarly to C₃ in distillationprocesses. Moreover, it is only slightly polar, so it can be difficultto remove by absorption. Accordingly, it would be beneficial to findadditional methods to more easily remove oxygenates from olefins,particularly from an olefin stream synthesized from methanol

U.S. Pat. Nos. 4,547,602 and 4,560,537 disclose a process for convertingmethanol to heavy hydrocarbons in the distillate range. In the firststage catalytic process, oxygenate is converted to lower olefins. Thefirst stage effluent is cooled in an exchanger to condense water and amajor amount of C5+ hydrocarbons. Byproduct water may be recovered fromunreacted feedstock and discarded or a portion may be recycled.

U.S. Pat. No. 4,387,263 discloses a process for making C2 to C4 olefinsfrom gas mixtures containing methanol, dimethyl ether. The reactiongasses, after condensation directs the reaction gasses and a condensedaqueous phase to a water scrubber in a water scrubbing column.

European Patent Application No. 0 060 103 discloses a process forconverting methanol and/or dimethyl ether to a product containingethylene. The product is condensed to form three phases—a liquid aqueousphase, a gaseous hydrocarbon phase and a liquid hydrocarbon phase. Theliquid aqueous phase contains unreacted methanol and dimethyl ether. Theliquid aqueous phase is stripped with low pressure steam to volatilizethe methanol and/or dimethyl ether to from the aqueous phase. Themethanol and/or dimethyl ether are recycled back to the reactor.Additionally, the gaseous hydrocarbon phase is contacted with methanolto selectively sorb unreacted dimethyl ether. An alternative contactsthe gaseous phase with water to selectively remove dimethyl ether.

SUMMARY OF THE INVENTION

This invention provides a process for producing olefins from methanolsuch that oxygenate contaminants can be more easily removed from theolefin product. In one embodiment, the invention comprises a method forprocessing an olefin stream containing oxygenates (comprising organiccompounds that contain at least one oxygen) and water. The methodcomprises providing an olefin stream containing oxygenates and water.The olefin stream is dewatered, and the dewatered olefin streamcompressed. The compressed olefin stream is then washed with methanol toremove at least a portion of the oxygenate from the olefin stream.Following methanol wash, the olefin stream is contacted with water, andthe water contacted olefin stream is fractionated.

In another embodiment, the water contacted olefin stream is dried priorto fractionating. In yet another embodiment, the washing of the olefinstream with the methanol and the water is carried out in a single washcolumn.

The invention also provides an embodiment, wherein the dewatered olefinstream is compressed to form a condensate containing dissolved heavyoxygenates and a gaseous olefin stream which is washed with themethanol. Desirably, the condensate is sent to a stripping column inwhich an overhead product of light hydrocarbons and a bottom product ofC₅+ hydrocarbons and heavy oxygenates are obtained. Preferably, theoverhead product of light hydrocarbons is sent to at least onecompression step.

In one embodiment, there is a method for producing olefins frommethanol. According to the method, a molecular sieve catalyst iscontacted with a first amount of methanol to produce an olefin stream.The olefin stream comprises an oxygenate. The oxygenate comprises anorganic compound that contains at least one oxygen. The olefin stream isdewatered. The dewatered olefin stream is compressed. Then, the olefinstream is washed with a second amount of methanol to remove at least aportion of the oxygenate from the olefin stream. The second amount isfrom 1% to 10% of the first amount. Next, the methanol washed olefinstream is contacted with water. Thereafter, the water contacted olefinstream is fractionated.

In another embodiment, the step of contacting a molecular sieve catalystwith a first amount of methanol occurs at a gas superficial velocitygreater than 1 m/s. In another embodiment, the step of contactingconverts from 90 wt. % to 98 wt. % of the first amount of methanol. Instill another embodiment, the step of contacting converts above 98 wt. %to less than 100 wt. % of the first amount of methanol.

BRIEF DESCRIPTION OF THE DRAWING

The attached FIGURE shows an example of but one type of flow scheme ofthe invention in which methanol is used to remove undesirable oxygenatesfrom an olefin stream synthesized from methanol.

DETAILED DESCRIPTION OF THE INVENTION

This invention provides a method for efficiently removing undesirableoxygenates from an olefin stream, particularly an olefin streamsynthesized from methanol. The method is achieved in an economical way.

According to the invention, oxygenates are washed out of the gaseousolefin product stream after at least one compression step, and beforethe olefin product is dried. The oxygenates are washed out in a washcolumn with methanol.

The invention is based on the fact that the drying steps that are calledfor in conventional ethylene plants are not capable of handling morethan very small amounts of oxygenates like aldehydes and ketones.Moreover, such oxygenates can polymerize under certain conditions, andthe polymerization reactions can reduce the active surfaces ofconventional drying schemes very rapidly. At present, there are no knowndryers that are suitable for handling such components. For this reason,the oxygenates have to be removed from the product stream before theyenter the cold part of the processing plant, which consists of olefindrying and fractionation.

One conceivable possibility for removing the oxygenates would be to useall of the fresh methanol that is provided for olefin synthesis to washthe oxygenates out of the olefin product stream before the olefinsynthesis. However, this would result in the olefin synthesis beingloaded with reaction products from the olefin synthesis. This can leadto the accumulation of undesirable components.

In this invention, the olefin stream containing the oxygenatecontaminants is quenched (dewatered), compressed and dried beforefractionation. The oxygenate removal step is desirably performed afterat least one compression step. If multiple compression steps are used,it is desirable that the oxygenate removal step be performed between thethird and fourth compression step. It is also desired that the oxygenateremoval step be performed after compression and before drying.

According to the invention, a wash column is provided for removing theoxygenate contaminant. Desirably, the oxygenates are washed out of thegaseous product stream with methanol in the wash column. Only arelatively small amount of methanol is needed for the oxygenate removal.For example, 1% to 10%, preferably 3% of the total amount of methanolused in the olefin synthesis, is used to wash out the oxygenates to anacceptable limit.

In another embodiment of the invention, overhead gas accumulating in themethanol wash is further washed with water. This removes methanol thatmay become entrained in the olefin stream as a result of evaporation inthe preceding methanol wash. The gaseous product stream that isrecovered from this water wash contains, besides the olefins, onlytraces of oxygenates and methanol. The water washed olefin productstream is then sent to drying and fractionation.

In yet another embodiment of the invention, the water wash is carriedout in an additional section of the wash column provided for themethanol wash. Desirably, a single wash column is provided for this,with the methanol wash being carried out in the lower part of the washcolumn, and the water wash in the upper part.

Another embodiment of the invention provides that the outlet pressure ofat least one of the compression steps be adjusted so that, optionallyafter cooling, a hydrocarbon-containing condensate in which heavyoxygenates are dissolved is formed in addition to the gaseous productstream. The condensate can be sent to a stripping column, in which anoverhead product of light hydrocarbons and a bottom product of C₅+hydrocarbons and heavy oxygenates is obtained. The overhead product oflight hydrocarbons is desirably sent to at least one compression step.The bottom product of heavy hydrocarbons and oxygenates can be sent tofurther processing.

The invention provides an economical method for removing oxygenates froman olefin-containing product stream, and enables effective oxygenseparation at very low investment cost. The invention is alsoadvantageous in that only a very small amount of methanol is needed toremove the oxygenates from the olefin stream and that the wash streamcan be used as feed to the olefin synthesis reactor. The olefinsynthesis reaction will not be adversely affected by the methanol washcontaining absorbed oxygenate. Nearly the same amount of fresh methanolis available for olefin synthesis. Desirably, the amount of methanolused in the methanol wash will be 1% to 10% of the total amount of themethanol that is used in the olefin synthesis reaction.

In one embodiment of the invention, the olefin product is obtained bycontacting methanol with an olefin forming catalyst in a reactor.Preferably, the catalyst is a molecular sieve catalyst.

Although the use of methanol to produce the olefin stream is preferred,other oxygenate components can be used as a feed. Such oxygenatescomprise at least one organic compound which contains at least oneoxygen atom, such as aliphatic alcohols, ethers, carbonyl compounds(aldehydes, ketones, carboxylic acids, carbonates, esters and the like).When the oxygenate is an alcohol, the alcohol includes an aliphaticmoiety having from 1 to 10 carbon atoms, more preferably from 1 to 4carbon atoms. Representative alcohols include but are not necessarilylimited to lower straight and branched chain aliphatic alcohols andtheir unsaturated counterparts. Examples of suitable oxygenate compoundsinclude, but are not limited to: methanol; ethanol; n-propanol;isopropanol; C₄-C₂₀ alcohols; methyl ethyl ether; dimethyl ether;diethyl ether, di-isopropyl ether; formaldehyde; dimethyl carbonate;dimethyl ketone; acetic acid; and mixtures thereof. Dimethyl ether, or amixture of dimethyl ether and methanol, are also preferred feeds.

Molecular sieves capable of converting an oxygenate such as methanol toan olefin compound include zeolites as well as non-zeolites, and are ofthe large, medium or small pore type. Small pore molecular sieves arepreferred in one embodiment of this invention, however. As definedherein, small pore molecular sieves have a pore size of less than 50 nm(5.0 angstroms). Generally, suitable catalysts have a pore size rangingfrom 35 nm (3.5 angstroms) to 50 nm (5.0 angstroms), preferably from 40nm (4.0 angstroms) to 50 nm (5.0 angstroms), and most preferably from 43nm (4.3 angstroms) to 50 nm (5.0 angstroms).

Zeolite materials, both natural and synthetic, have been demonstrated tohave catalytic properties for various types of hydrocarbon conversionprocesses. In addition, zeolite materials have been used as adsorbents,catalyst carriers for various types of hydrocarbon conversion processes,and other applications. Zeolites are complex crystallinealuminosilicates which form a network of AlO₂ ⁻ and SiO₂ tetrahedralinked by shared oxygen atoms. The negativity of the tetrahedra isbalanced by the inclusion of cations such as alkali or alkaline earthmetal ions. In the manufacture of some zeolites, non-metallic cations,such as tetramethylammonium (TMA) or tetrapropylammonium (TPA), arepresent during synthesis. The interstitial spaces or channels formed bythe crystalline network enable zeolites to be used as molecular sievesin separation processes, as catalyst for chemical reactions, and ascatalyst carriers in a wide variety of hydrocarbon conversion processes.

Zeolites include materials containing silica and optionally alumina, andmaterials in which the silica and alumina portions have been replaced inwhole or in part with other oxides. For example, germanium oxide, tinoxide, and mixtures thereof can replace the silica portion. Boron oxide,iron oxide, gallium oxide, indium oxide, and mixtures thereof canreplace the alumina portion. Unless otherwise specified, the terms“zeolite” and “zeolite material” as used herein, shall mean not onlymaterials containing silicon atoms and, optionally, aluminum atoms inthe crystalline lattice structure thereof, but also materials whichcontain suitable replacement atoms for such silicon and aluminum atoms.

One type of olefin forming catalyst capable of producing largequantities of ethylene and propylene is a silicoaluminophosphate (SAPO)molecular sieve. Silicoaluminophosphate molecular sieves are generallyclassified as being microporous materials having 8, 10, or 12 memberedring structures. These ring structures can have an average pore sizeranging from 35 nm (3.5 angstroms) to 150 nm (15 angstroms). Preferredare the small pore SAPO molecular sieves having an average pore size ofless than 50 nm (5 angstroms), preferably an average pore size rangingfrom 35 nm (3.5 angstroms) to 50 nm (5 angstroms), more preferably from35 nm (3.5 angstroms) to 42 nm (4.2 angstroms). These pore sizes aretypical of molecular sieves having 8 membered rings.

According to one embodiment, substituted SAPOs can also be used inoxygenate to olefin reaction processes. These compounds are generallyknown as MeAPSOs or metal-containing silicoaluminophosphates. The metalcan be alkali metal ions (Group IA), alkaline earth metal ions (GroupIIA), rare earth ions (Group IIIB, including the lanthanoid elements:lanthanum, cerium, praseodymium, neodymium, samarium, europium,gadolinium, terbium, dysprosium, holmium, erbium, thulium, ytterbium andlutetium; and scandium or yttrium) and the additional transition cationsof Groups IVB, VB, VIB, VIIB, VIIIB, and IB.

Preferably, the Me represents atoms such as Zn, Mg, Mn, Co, Ni, Ga, Fe,Ti, Zr, Ge, Sn, and Cr. These atoms can be inserted into the tetrahedralframework through a [MeO₂] tetrahedral unit. The [MeO₂] tetrahedral unitcarries a net electric charge depending on the valence state of themetal substituent. When the metal component has a valence state of +2,+3, +4, +5, or +6, the net electric charge is between −2 and +2.Incorporation of the metal component is typically accomplished addingthe metal component during synthesis of the molecular sieve. However,post-synthesis ion exchange can also be used. In post synthesisexchange, the metal component will introduce cations into ion-exchangepositions at an open surface of the molecular sieve, not into theframework itself.

Suitable silicoaluminophosphate molecular sieves include SAPO-5, SAPO-8,SAPO-11, SAPO-16, SAPO-17, SAPO-18, SAPO-20, SAPO-31, SAPO-34, SAPO-35,SAPO-36, SAPO-37, SAPO-40, SAPO-41, SAPO-42, SAPO-44, SAPO-47, SAPO-56,the metal containing forms thereof, and mixtures thereof. Preferred areSAPO-18, SAPO-34, SAPO-35, SAPO-44, and SAPO-47, particularly SAPO-18and SAPO-34, including the metal containing forms thereof, and mixturesthereof. As used herein, the term mixture is synonymous with combinationand is considered a composition of matter having two or more componentsin varying proportions, regardless of their physical state.

An aluminophosphate (ALPO) molecular sieve can also be included in thecatalyst composition. Aluminophosphate molecular sieves are crystallinemicroporous oxides which can have an AlPO₄ framework. They can haveadditional elements within the framework, typically have uniform poredimensions ranging from 30 nm (3 angstroms) to 100 nm (10 angstroms),and are capable of making size selective separations of molecularspecies. More than two dozen structure types have been reported,including zeolite topological analogues. A more detailed description ofthe background and synthesis of aluminophosphates is found in U.S. Pat.No. 4,310,440, which is incorporated herein by reference in itsentirety. Preferred ALPO structures are ALPO-5, ALPO-11, ALPO-18,ALPO-31, ALPO-34, ALPO-36, ALPO-37, and ALPO-46.

The ALPOs can also include a metal substituent in its framework.Preferably, the metal is selected from the group consisting ofmagnesium, manganese, zinc, cobalt, and mixtures thereof. Thesematerials preferably exhibit adsorption, ion-exchange and/or catalyticproperties similar to aluminosilicate, aluminophosphate and silicaaluminophosphate molecular sieve compositions. Members of this class andtheir preparation are described in U.S. Pat. No. 4,567,029.

The metal containing ALPOs have a three-dimensional microporous crystalframework structure of MO₂, AlO₂ and PO₂ tetrahedral units. These asmanufactured structures (which contain template prior to calcination)can be represented by empirical chemical composition, on an anhydrousbasis, as:mR: (M_(x)Al_(y)P_(z))O₂wherein “R” represents at least one organic templating agent present inthe intracrystalline pore system; “m” represents the moles of “R”present per mole of (M_(x)Al_(y)P_(z))O₂ and has a value of from zero to0.3, the maximum value in each case depending upon the moleculardimensions of the templating agent and the available void volume of thepore system of the particular metal aluminophosphate involved, “x”, “y”,and “z” represent the mole fractions of the metal “M”, (i.e. magnesium,manganese, zinc and cobalt), aluminum and phosphorus, respectively,present as tetrahedral oxides.

The metal containing ALPOs are sometimes referred to by the acronym asMeAPO. Also in those cases where the metal “Me” in the composition ismagnesium, the acronym MAPO is applied to the composition. SimilarlyZAPO, MnAPO and CoAPO are applied to the compositions which containzinc, manganese and cobalt respectively. To identify the variousstructural species which make up each of the subgeneric classes MAPO,ZAPO, CoAPO and MnAPO, each species is assigned a number and isidentified, for example, as ZAPO-5, MAPO-11, CoAPO-34 and so forth.

The silicoaluminophosphate molecular sieve is typically admixed (i.e.,blended) with other materials. When blended, the resulting compositionis typically referred to as a SAPO catalyst, with the catalystcomprising the SAPO molecular sieve.

Materials which can be blended with the molecular sieve can be variousinert or catalytically active materials, or various binder materials.These materials include compositions such as kaolin and other clays,various forms of rare earth metals, metal oxides, other non-zeolitecatalyst components, zeolite catalyst components, alumina or aluminasol, titania, zirconia, magnesia, thoria, beryllia, quartz, silica orsilica or silica sol, and mixtures thereof. These components are alsoeffective in reducing, inter alia, overall catalyst cost, acting as athermal sink to assist in heat shielding the catalyst duringregeneration, densifying the catalyst and increasing catalyst strength.It is particularly desirable that the inert materials that are used inthe catalyst to act as a thermal sink have a heat capacity of from 0.05to 1 cal/g-° C., more preferably from 0.1 to 0.8 cal/g-° C., mostpreferably from 0.1 to 0.5 cal/g-° C.

Additional molecular sieve materials can be included as a part of theSAPO catalyst composition or they can be used as separate molecularsieve catalysts in admixture with the SAPO catalyst if desired.Structural types of small pore molecular sieves that are suitable foruse in this invention include AEI, AFT, APC, ATN, ATT, ATV, AWW, BIK,CAS, CHA, CHI, DAC, DDR, EDI, ERI, GOO, KFI, LEV, LOV, LTA, MON, PAU,PHI, RHO, ROG, THO, and substituted forms thereof. Structural types ofmedium pore molecular sieves that are suitable for use in this inventioninclude MFI, MEL, MTW, EUO, MIT, HEU, FER, AFO, AEL, TON, andsubstituted forms thereof. These small and medium pore molecular sievesare described in greater detail in the Atlas of Zeolite StructuralTypes, W. M. Meier and D. H. Olsen, Butterworth Heineman, 3rd ed., 1997,the detailed description of which is explicitly incorporated herein byreference. Preferred molecular sieves which can be combined with asilicoaluminophosphate catalyst include ZSM-5, ZSM-34, erionite, andchabazite.

The catalyst composition, according to an embodiment, preferablycomprises from 1% to 99%, more preferably from 5% to 90%, and mostpreferably from 10% to 80%, by weight of molecular sieve. It is alsopreferred that the catalyst composition have a particle size of from 0.2μm (20 angstroms) to 30 μm (3,000 angstroms), more preferably from 0.3μm (30 angstroms) to 2 μm (200 angstroms), most preferably from 0.5 μm(50 angstroms) to 1.5 μm (150 angstroms).

The catalyst can be subjected to a variety of treatments to achieve thedesired physical and chemical characteristics. Such treatments include,but are not necessarily limited to hydrothermal treatment, calcination,acid treatment, base treatment, milling, ball milling, grinding, spraydrying, and combinations thereof.

A molecular sieve catalyst particularly useful in making ethylene andpropylene is a catalyst which contains a combination of SAPO-34, andSAPO-18 or ALPO-18 molecular sieve. In a particular embodiment, themolecular sieve is a crystalline intergrowth of SAPO-34, and SAPO-18 orALPO-18.

To convert methanol or other oxygenate to olefin, conventional reactorsystems can be used, including fixed bed, fluid bed or moving bedsystems. Preferred reactors of one embodiment are co-current riserreactors and short contact time, countercurrent free-fall reactors.Desirably, the reactor is one in which an oxygenate feedstock can becontacted with a molecular sieve catalyst at a weight hourly spacevelocity (WHSV) of at least 1 h⁻¹, preferably in the range of from 1hr⁻¹ to 1000 hr⁻¹, more preferably in the range of from 20 hr⁻¹ to 1000hr⁻¹, and most preferably in the range of from 50 hr⁻¹ to 500 hr⁻¹. WHSVis defined herein as the weight of oxygenate, and reactive hydrocarbonwhich may optionally be in the feed, per hour per weight of themolecular sieve in the reactor. Because the catalyst or the feedstockmay contain other materials which act as inerts or diluents, the WHSV iscalculated on the weight basis of the oxygenate feed, and any reactivehydrocarbon which may be present with the oxygenate feed, and themolecular sieve contained in the reactor.

Preferably, the oxygenate feed is contacted with the catalyst when theoxygenate is in a vapor phase. Alternately, the process may be carriedout in a liquid or a mixed vapor/liquid phase. When the process iscarried out in a liquid phase or a mixed vapor/liquid phase, differentconversions and selectivities of feed-to-product may result dependingupon the catalyst and reaction conditions.

The process can generally be carried out at a wide range oftemperatures. An effective operating temperature range can be from 200°C. to 700° C., preferably from 300° C. to 600° C., more preferably from350° C. to 550° C. At the lower end of the temperature range, theformation of the desired olefin products may become markedly slow with arelatively high content of oxygenated olefin by-products being found inthe olefin product. However, the selectivity to ethylene and propyleneat reduced temperatures may be increased. At the upper end of thetemperature range, the process may not form an optimum amount ofethylene and propylene product, but the conversion of oxygenate feedwill generally be high.

Operating pressure also may vary over a wide range, including autogenouspressures. Effective pressures include, but are not necessarily limitedto, a total pressure of at least 1 psia (7 kPa), preferably at least 5psia (34 kPa). The process is particularly effective at higher totalpressures, including a total pressure of at least 20 psia (138 kPa).Preferably, the total pressure is at least 25 psia (172 kPa), morepreferably at least 30 psia (207 kPa). For practical design purposes itis desirable to use methanol as the primary oxygenate feed component,and operate the reactor at a pressure of not greater than 500 psia (3445kPa), preferably not greater than 400 psia (2756 kPa), most preferablynot greater than 300 psia (2067 kPa).

Undesirable by-products can be avoided by operating at an appropriategas superficial velocity. As the gas superficial velocity increases theconversion decreases avoiding undesirable by-products. As used herein,the term, “gas superficial velocity” is defined as the combinedvolumetric flow rate of vaporized feedstock, which includes diluent whenpresent in the feedstock, as well as conversion products, divided by thecross-sectional area of the reaction zone. Because the oxygenate isconverted to a product having significant quantities of ethylene andpropylene while flowing through the reaction zone, the gas superficialvelocity may vary at different locations within the reaction zone. Thedegree of variation depends on the total number of moles of gas presentand the cross section of a particular location in the reaction zone,temperature, pressure and other relevant reaction parameters.

In one embodiment, the gas superficial velocity is maintained at a rateof greater than 1 meter per second (m/s) at least one point in thereaction zone. In another embodiment, it is desirable that the gassuperficial velocity is greater than 2 m/s at least one point in thereaction zone. More desirably, the gas superficial velocity is greaterthan 2.5 m/s at least one point in the reaction zone. Even moredesirably, the gas superficial velocity is greater than 4 m/s at leastone point in the reaction zone. Most desirably, the gas superficialvelocity is greater than 8 m/s at least one point in the reaction zone.

According to yet another embodiment of the invention, the gassuperficial velocity is maintained relatively constant in the reactionzone such that the gas superficial velocity is maintained at a rategreater than 1 m/s at all points in the reaction zone. It is alsodesirable that the gas superficial velocity be greater than 2 m/s at allpoints in the reaction zone. More desirably, the gas superficialvelocity is greater than 2.5 m/s at all points in the reaction zone.Even more desirably, the gas superficial velocity is greater than 4 m/sat all points in the reaction zone. Most desirably, the gas superficialvelocity is greater than 8 m/s at all points in the reaction zone.

The amount of ethylene and propylene produced in the oxygenate to olefinprocess can be increased by reducing the conversion of the oxygenates inthe oxygenate to olefins reaction. However, reducing the conversion offeed oxygenates in the oxygenate conversion reaction tends to increasethe amount of oxygenated hydrocarbons, particularly including dimethylether, that are present in the olefin product. Thus, control of theconversion of feed to the oxygenate reaction process can be important.

According to one embodiment, the conversion of the primary oxygenate,e.g., methanol, is from 90 wt % to 98 wt %. According to anotherembodiment the conversion of methanol is from 92 wt % to 98 wt %,preferably from 94 wt % to 98 wt %.

According to another embodiment, the conversion of methanol is above 98wt % to less than 100 wt %. According to another embodiment, theconversion of methanol is from 98.1 wt % to less than 100 wt %;preferably from 98.2 wt % to 99.8 wt %. According to another embodiment,the conversion of methanol is from 98.2 wt % to less than 99.5 wt;preferably from 98.2 wt % to 99 wt %.

In this invention, weight percent conversion is calculated on a waterfree basis unless otherwise specified. Weight percent conversion on awater free basis is calculated as: 100×(weight oxygenate fed on a waterfree basis−weight oxygenated hydrocarbon in the product on a water freebasis). The water free basis of oxygenate is calculated by subtractingout the water portion of the oxygenate in the feed and product, andexcluding water formed in the product. For example, the weight flow rateof methanol on an oxygenate free basis is calculated by multiplying theweight flow rate of methanol by 14/32 to remove the water component ofthe methanol. As another example, the rate flow rate of dimethyl etheron an oxygenate free basis is calculated by multiplying the weight flowrate of dimethylether by 28/46 to remove the water component of thedimethyl ether. If there is a mixture of oxygenates in the feed orproduct, trace oxygenates are not included. When methanol and/ordimethyl ether is used as the feed, only methanol and dimethyl ether areused to calculate conversion on a water free basis.

In this invention, selectivity is also calculated on a water free basisunless otherwise specified. Selectivity is calculated as: 100×wt %component/(100−wt % water−wt % methanol−wt % dimethyl ether) whenmethanol and/or dimethyl ether is used as the feed.

An example of the invention is shown in the Figure. According to theFigure, an olefin product stream containing, in addition to ethylene andpropylene, longer chain olefins and oxygenates, is sent from a reactorfor production of olefins from methanol (not shown in the figure) via apipe 1 to a gas-liquid separation device 2. Condensate, whichaccumulates in the gas-liquid separation device 2, and contains smallamounts of hydrocarbons, is sent via a pipe 3 to a three-phase separator4. Gas and water that accumulate in the three-phase separator are sentback to gas compressors via pipes 6 and 7. A large quantity of heavyoxygenates is dissolved in the remaining hydrocarbon-containingcondensate. This condensate is sent to a stripping column 5.

Overhead gas that forms in the stripping column 5 is sent to gascompressors via a pipe 8. Heavy C₅+ hydrocarbons in which heavyoxygenates are dissolved are removed as bottom product via a pipe 9.This bottom by-product stream can be sent to further processing.

The olefin product stream in the gas-liquid separation device 2 is sentvia a pipe 10 to the bottom portion of a wash column 12, after slightheating by means of heat exchanger 11. The oxygenates are washed out ofthe olefin product stream in a lower part 13 of the wash column 12 usingfresh methanol. The fresh methanol is sent to the wash column 12 via apipe 15.

Overhead gas from the methanol wash is sent to an additional wash, forwhich an upper part 14 of the wash column is provided. There, theoverhead gas, which can contain evaporated methanol from the methanolwash is contacted with fresh water, which is sent to the wash column 12via a pipe 16. The overhead gas exiting the upper part 14 of the washcolumn 12 contains only tolerable traces of oxygenates and methanol.

The overhead gas exiting the upper part 14 is sent via a pipe 17. Thisgas can be sent, for example, to an alkali wash and to a drying step.Then fractionation of the olefin (not shown in the Figure) is carriedout.

The bottom product of the water wash is sent partly through the lowerpart 13 of the wash column 12, and partly to a flash drum 20 as by-passvia a pipe 19. The gas accumulating in flash drum 20 containspredominantly ethylene and propylene, and is sent back to the processvia the compression steps. The liquid accumulating in flash drum 20 is amethanol-water solution that contains the oxygenates in a highconcentration. This liquid is removed via a pipe 22, and can be sent toincineration or otherwise utilized. Alternatively, this solution can besent to a methanol-water distillation separation, where the oxygenatesremain in the distillate. The solution can also be returned to theprocess together with the methanol. Through other separation steps, itis also possible to obtain clean methanol and to discard the oxygenate.In the case of a light waste gas, it may be meaningful to choose thepressure in pipe 1 after the compression steps so that no hydrocarboncondensate accumulates. In this case, the three-phase separator 4 andthe stripping column 5 can be omitted.

1. A method for producing olefins from methanol, the method comprisingthe steps of: contacting a molecular sieve catalyst with a first amountof methanol to produce an olefin stream, comprising an oxygenate whereinthe oxygenate comprises an organic compound that contains at least oneoxygen; removing oxygenates from the olefin stream by the stepsconsisting of: dewatering the olefin stream; compressing the dewateredolefin stream one or more times; washing the dewatered olefin streamwith a second amount of methanol to remove at least a portion of theoxygenate from the olefin stream, wherein the second amount is from 1%to 10% of the first amount; contacting the methanol washed olefin streamwith water; drying the water contacted olefin stream; and fractionatingthe dried olefin stream to produce an olefin stream.
 2. The method ofclaim 1, wherein the washing the olefin stream with methanol to removeat least a portion of the oxygenate from the olefin stream; and thecontacting the methanol washed olefin stream with water is carried outin a single wash column.
 3. The method of claim 1, wherein the dewateredolefin stream is compressed to form a condensate containing dissolvedheavy oxygenates and a gaseous olefin stream which is washed with themethanol.
 4. The method of claim 3, further comprising sending thecondensate to a stripping column in which an overhead product of lighthydrocarbons and a bottom product of C₅+ hydrocarbons and heavyoxygenates are obtained.
 5. The method of claim 4, wherein the overheadproduct of light hydrocarbons is sent to at least one compression step.6. The process of claim 1, wherein the step of contacting the molecularsieve catalyst occurs at a gas superficial velocity greater than 1 m/s.7. The process of claim 1, wherein the step of contacting the molecularsieve catalyst converts from 90 wt. % to 98 wt. % of the first amount ofmethanol.
 8. The process of claim 1, wherein the step of contactingconverts above 98 wt. % to less than 100 wt. % of the first amount ofmethanol.